Rohan says diamonds are just coal that was squeezed hard enough underground for long enough

Why

Rohan's idea is repeated so often it is almost taken for granted, and it has a neat logic — coal is carbon, diamond is carbon, and pressure turns soft things hard. But the timeline rules it out. Coal forms from the buried remains of land plants, and the earliest land plants are a few hundred million years old, while most natural diamonds are between one and three billion years old — they already existed long before there were any plants to become coal. Diamonds form from carbon much deeper in the Earth, in the mantle, roughly 150 kilometres down, far deeper than coal ever forms, and are carried up by rare violent eruptions. Test: look up when land plants first appeared against the measured ages of diamonds — the diamonds are far older. Nisha is right: diamonds and coal are both carbon, but diamonds do not come from coal — they are older than the plants that coal is made from.